The present invention relates to a message transmission system, to a method of operating the message transmission system and to a primary station therefor. The invention has particular application to point-to-point telecommunications systems such as digital wide area paging and cellular and cordless telephone systems.
For convenience of description the present invention will be described to digital wide area paging with particular reference being made to the well known CCIR Radiopaging Code No. 1, otherwise known as POCSAG. However it must be understood that the present invention is independent of any specific code or protocol. POCSAG has provision for transmitting numeric only and alpha-numeric messages and a typical transmission comprises an address code word and one or more concatenated message code words. Address and message code words are 32 bits long. In the case of a message code word only 20 of the 32 bits are allocated to data. If a numeric only message is being transmitted then each digit is encoded as a 4 bit hexadecimal character so that each message code word can comprise a maximum of 5 digits, which means that to transmit an eleven digit trunk code or telephone number requires a total of 3 message code words plus an address code word. In the case of an alpha-numeric message, each digit, letter or space is encoded as ASCII seven bit character which means that only 2 complete characters plus 6 bits of a third character or 6 bits comprising the end of one character and the beginning of another character can be sent in a single message code word.
In the interests of maximising system capacity Patent Specification WO95/02873 discloses a method of message compression in which a paging system controller (or primary station) has a dictionary stored in a non-volatile memory and each pager (or secondary station) has the identical dictionary stored in it. In operation if the pager system controller detects that one or more words contained in a message to be transmitted are in the dictionary then rather than encode the or each word using ASCII characters, the location of the word in the dictionary is sent as say 2 seven bit characters. The pager on decoding these two characters, reads out the word from its dictionary and substitutes it for the two characters. This principle can be extended to standard phrases, such as "for example", and frequently used short messages. Compression increases the overall system capacity.
A typical numeric only pager message is an indication of a telephone number which the pager user is being requested to ring. Instead of having to transmit each digit of a telephone number as 4 bit hexadecimal character it has been proposed that the ten most frequently requested telephone numbers that a pager user is required to call are prestored in a non-volatile memory in the pager, each number being identifiable by a memory location reference indication. The paging system controller also has a non-volatile memory in which for the same pager, the same telephone numbers and location reference indicia are stored. Thus when the pager system controller calls one of these prestored numbers, all it has to do is to transmit the address code word and one concatenated message code word giving the memory location reference indicia. At the pager the pre-stored telephone number is read-out and displayed. On the basis of a simple comparison of requiring 4 code words to send an address and a telephone number (as 4 bit hexadecimal) and 2 code words to send an address and a telephone number in a compressed form, the capacity of the system is doubled.
A drawback to the system as described is that it is inflexible in so far that if due to a change in circumstances a user changes his frequently used work telephone number, the paging system controller has to send the new number in an uncompressed form. If there are a lot of changes across the entire system, then the value of the compressed format diminishes. Also in the case of alpha-numeric data, it is difficult and relatively expensive to produce custom dictionaries for each individual user.